1. Field of the Invention
The invention pertains to a process for anodic or cathodic electrocoating of strip or profile material.
2. Description of Related Art
A similar process is described with reference to electrocoating of strip material in U.S. Pat. Nos. 4,007,102 and 4,175,018.
The strip material is unwound from a coil, runs over guide rollers through a series of cleaning and rinsing tanks and is then conveyed over additional guide rollers into a tank in which the electrocoating takes place. Electrocoating using a water-soluble paint as the electrolytic liquid is based on the physical principle of electrophoresis. The electrically conductive strip material, aluminum or steel strip, for instance, is directed through the electrocoating bath, whereby the paint is deposited on the metallic substrate during dip-coating by means of electrochemical and chemical reactions. The water-soluble paint functioning as the electrolytic liquid contains as a binder groups which can be ionized and form salts, these being in fact insoluble in water in this form, but which, if these binders contain groups which are acidic in character (carboxyl groups), can be made water soluble in a neutralization reaction using alkaline media, such as amines, whereby salts are formed.
In order to avoid solid particles settling out, the baths must be continuously recirculated; the particles amenable to deposition are in addition transported in this way.
When a DC voltage is applied the phenomenon known as electrophoresis, i.e. the migration of charged particles to the anode or to the cathode, is invoked. If the paint particles in an aqueous system carry a positive charge, i.e. if the binders contain alkaline groups and have been neutralized with acids, they coagulate in the alkaline environment while forming a film on the cathode. The strip material to be painted represents the cathode in this case and one refers to cathodic electro-dip coating. In contrast, the paint particles will coagulate and form a film on the anode if they have been neutralized with amines and exhibit a negative charge in the aqueous system. In this case the workpiece represents the anode and one refers to anodic electro-dip coating.
Using the continuous process described in the above-mentioned U.S. patents makes it possible to coat strip material at high speed and great uniformity. Once the strip has left the electrolyte bath any excess electrolyte which has not coagulated is rinsed off in a subsequent bath, whereafter the coating is dried in a dryer unit.
With the known process incorporating continuous passage through a submersion bath it is only possible to coat both surfaces of the material simultaneously and with layers of identical thicknesses. Furthermore with this process allows only for coating flat strip material, not profiled strip.